Protection circuit

ABSTRACT

A protection circuit for protecting circuit elements (TX) connected to line via terminal means (L1 and L2) from the effects of a transient abnormal high voltage surge, comprising a semiconductor switch (TR1 and TR2), a first circuit comprising a resistor (R1 ) in series with the conductor path of the semiconductor switch and a voltage reference means (D1, and D2)in parallel with the resistor and a junction of the semiconductor, and a second circuit comprising the first voltage reference means in series with a second voltage reference means (D3), the second circuit being connected in parallel with the resistor. The second circuit further includes a voltage dependent resistance (VC1). When the current through the resistor exceeds normal line current the voltage reference means (D1, D2, D3) clamp the voltage drop across the resistor and turn off the semiconductor switch.

TECHNICAL FIELD

The invention relates to a protective circuit arrangement for protectingcircuit elements connected to a line against the damaging effects oftransient abnormal high voltage surges on the line caused by, forexample, lightning or fluctuations in adjacent power transmission lines.The circuit arrangement of the present invention is particularly, thoughnot exclusively, suitable for protecting line associated elements of atelephone subset connected to an exchange line via a solid state lineswitch, the line switch providing a hook-switch function andloop-current interrupt functions.

Protection circuits for telephone subsets of the aforementioned kind areknown and comprises a voltage limiting circuit and a current limitingcircuit. The voltage limiting circuit comprises a voltage dependentnonlinear resistor connected across the subset's line terminals tosubstantially absorb transient high voltage surges on the line, thusprotecting circuit elements in the subset that are associated with theline terminal end of the subset's circuit. In addition to the voltagelimiting circuit, a current limiting circuit is also provided. Thecurrent limiting circuit comprises an arrangement of a resistor inseries with and on the line side of the conductive path of the lineswitch, and a diode arrangement connected between the line end of theresistor and the line switch's control element. The value of theresistor is chosen so that when a current threshold is exceeded, thevoltage drop across the resistor exceeds the forward drop across thediode arrangement thus limiting the current through the conductive pathof the line switch.

A disadvantage of the known protection circuit is that a proportion ofthe current produced by the transient voltage surge passes through theline switch and therefore the line switch's power rating must besufficient to accommodate this transient current and the power ratingchosen accordingly. This is reflected in the cost of the transistorsused in the line switch.

It is therefore an object of the present invention to provide animproved protection circuit for protecting electrical equipmentconnected to a line via a solid state switch from the damaging effectsof the transient abnormal voltage surges on the line, and therebypermitting the power rating of the transistors used in the line switchto be lowered.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided aprotection circuit for protecting circuit elements connected to a linevia line terminal means from the effects of a transient abnormal highvoltage surge, said circuit comprising a controllable semiconductorswitch having a conductive path is serially connected between said lineterminal means and said circuit elements for selectively connecting saidcircuit elements to said line, said protection circuit comprising afirst circuit comprising a first resistance means of a predeterminedvalue connected in series between one terminal of the line terminalmeans and the conductive path of the said semiconductor switch, and afirst voltage reference means connected in parallel with said firstresistance means and the junction of the said semiconductor's conductivepath its control element, and a second circuit comprising the firstvoltage reference means in series with a second voltage reference means,said second circuit being connected in parallel with said firstresistance means, said second circuit further including a voltagedependent second resistance means having a predetermined threshold valueconnected between the junction of the first and resistance means and theconductive path of the said semiconductor switch and another terminal ofsaid line terminal means, the resistance value of said first resistancemeans being chosen so that with a normal magnitude of line current thesum of the voltage drops across the first resistance means and the saidjunction of the semiconductor's conductive path and its control elementis less than the reference voltage of the first reference means, andwhen the magnitude of the line current exceeds the normal magnitude, thesaid sum of the voltage drops increases accordingly until it reaches thereference voltage of the first reference voltage means whereupon saidreference voltage means clamps the voltage across said first resistancemeans and thereby limits the current therethrough, and upon said voltagedependent second resistance means detecting its threshold voltage itpermits current to increase through said first resistance means untilthe voltage drop thereon equals the sum of the reference voltages of thefirst and second reference voltage means whereupon the voltage acrosssaid first resistor means is clamped thereby and the semiconductorswitch means rendered nonconducting by the reference voltage of saidsecond voltage reference means.

According to a further aspect of the present invention there is provideda protection circuit wherein said first voltage reference meanscomprises two serially connected semiconductor diodes.

BRIEF DESCRIPTION OF DRAWINGS

In order that the invention may be carried into effect, embodimentsthereof will now be described in relation to the drawings, in which:

FIG. 1 shows part of the line circuit of a telephone subsetincorporating a known protection circuit;

FIG. 2 shows part of the line circuit of a telephone subsetincorporating the protection circuit of the present invention; and

FIG. 3 shows a circuit of an AC power switching arrangementincorporating the protection circuit of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

Referring to FIG. 1, the circuit comprises line terminals L1 and L2 forconnection to an exchange line (not shown). Across terminals L1 and L2is connected a voltage protection varistor VC1. A line switch in theform of a complementary pair of transistors comprising an NPNtransistors TR1 and a PNP transistor TR2 is serially connected betweenthe line terminals and the subset's transmission circuit TX. Thecollector/emitter junction of a line switch control transistor TR3 isconnected between the base element of transistor TR2 and terminal L2. Aresistor R2 is connected between the base of transistor TR2 and thecollector element of transistor TR3 to provide a small but sufficientbase current for transistors TR2. The base element of transistor TR3 iscoupled to an output of a micro-processor (not shown) in thetransmission circuit. Signals at this output control the functions ofthe line switch in a known manner. A resistor R1 is serially connectedbetween terminal L1 and the collector/emitter junction of transistorTR1, and a serial arrangement of two diodes D1 and D2 are connectedbetween terminal L1 and the base element of transistor TR2. Resistor R1and diodes D1 and D2 form the current limiting section of the protectioncircuit. The operation of this known current protector circuit has beendescribed above.

Referring to FIG. 2, except for the protection circuit, the circuit isidentical to that of FIG. 1 and will not be described. The protectioncircuit shown in FIG. 2 comprises resistor R1 serially connected betweenline terminals L1 and the collector/emitter junction of transistor TR1;diodes D1 and D2 serially connected between terminal L1 and the baseelement of transistor TR2; a varistor VC1 connected between the junctionof resistor R1 and the emitter of transistor TR2, and terminal L2; and afurther diode D3 connected across the emitter and base of transistorTR2. The resistance value of resistor R1 is chosen so that the voltagedrop across it at normal operating line current (≦150 ma) is less thanthe forward bias voltage of diode D1. Typically, the resistance ofresistor R1 is approximately 3 ohma.

In operation, with normal line current flowing through resistor R1 andthe saturated line switch, the forward bias voltage across thebase/emitter junction of transistor TR2 is 0.6 volts and the voltagedrop across resistor R1 is less than 0.6 volts. Therefore the voltagedrop across diodes D1 and D2 is the sum of the forward bias voltageacross the base/emitter junction of transistor TR2 and the voltage dropacross resistor R1, which when normal line current is flowing is lessthan the sum of the forward bias voltages of diodes D1 and D2 (1.2volts).

In the event of a high voltage surge on the line, the line currentbegins to increase causing the voltage drop across resistor R1 toincrease accordingly; the voltage across the base/emitter junction oftransistor TR2, of course, does not change. When the sum of the voltagedrops across resistor R1 and the base/emitter junction exceeds the sumof the forward bias voltages of diodes D1 and D2, the latter becomeforward biased. The forwardly biased diodes clamp the voltage acrossresistor R1 and the base/emitter junction of transistor TR2 and henceprevent an increase in base drive current to transistor TR2, therebylimiting the current through transistor TR1.

At the same time the voltage across varistor VC1 is rising and uponreaching the operating value of varistor VC1, typically 180-250 volts,current through resistor R1 increases until the sum of the voltage dropsacross resistor R1 and base/emitter junction of transistor TR2 exceedsthe sum of the forward bias voltages of diodes D1, D2 and D3. A currentpath is now established from line terminal L1, diodes D1, D2, D3,varistor VC1 to line terminal L2. The forward bias voltage of diode D3reverse bias' transistor TR2 which turns off, thereby turning offtransistor TR1 and pre-venting virtually any current from flowingthrough the line switch during the remainder of the voltage surge.

Preferably, a polarity guard circuit (not shown) is interposed betweenthe line terminals and the line switch to ensure that the polarity ofthe surge voltage is correctly presented to the diodes of the protectioncircuit.

Referring to FIG. 3, the AC power switching circuit comprises an ACinput (˜) coupled to a load RL via a pair of solid state switchingcircuits each incorporating the protection circuit according to thepresent invention to provide full-wave protection in the event of anabnormal power surge on a line (not shown) connected to the input.Varistor VC1 is common to both protection circuits. Switch contacts S1and S2 when activated switch transistors TR3 and TR23 which in turnswitch on the line switch transistors TR1, TR2 and TR21, TR22.

In operation, during the first half cycle the current path for normalline current is via resistor R1, collector/emitter junction ortransistor TR1, load RL, diode D24 to line. The elements of theprotection circuit have the same reference labels as in FIG. 2. If avoltage surge occurs it will cause varistor VC1 to conduct and theresulting forward bias voltage of diode D3 will reverse bias transistorTR2 which will turn off and hence turn off transistor TR1 preventingcurrent from flowing into the load.

During the second half cycle the current path for normal line current isresistor R21, collector/emitter junction of transistor TR21, load RL,diode D4, to line. When the voltage across varistor VC1 exceeds itsthreshold and begins to conduct, the resulting forward bias voltage ofdiode D23 will reverse bias transistor TR22 which will turn off and turnoff transistor TR21. Diodes D5, D6, D25 and D26 divert the surge currentaway from the load RL.

While the present invention has been described with regard to manyparticulars it is understood that equivalents may be readily substitutedwithout departing from the scope of the invention.

INDUSTRIAL APPLICABILITY

The above described invention may be advantageously applied in themanufacture of telephone subset apparatus.

I claim:
 1. A protection circuit for protecting circuit elementsconnected to a line via line terminal means from the effects of atransient abnormal high voltage surge, said circuit comprisingacontrollable semiconductor switch having a conductive path seriallyconnected between said line terminal means and said circuit elements andresponsive to an associated control element for selectively connectingsaid circuit elements to said line, a connection between said conductivepath and said control element defining a semiconductor junction, a firstcircuit further comprisingfirst resistance means of a predeterminedvalue connecting one terminal of the line terminal means with theconductive path of the semiconductor switch and connected in series withsaid semiconductor junction, and first voltage reference means connectedin parallel with said first resistance means and said semiconductorjunction, and a second circuit comprisingthe first voltage referencemeans, second voltage reference means in series with said first voltagereference means, said first and second voltage reference means beingconnected in parallel with said first resistance means, and voltagedependent second resistance means having a predetermined threshold valueand connected between the conductive path and another terminal of saidline terminal means,wherein the resistance value of said firstresistance means is such that with a magnitude of line current within apredetermined normal range, the sum of the voltage drops across thefirst resistance means and said semiconductor junction is less than thereference voltage of the first reference means, and as the magnitude ofthe line current exceeds said predetermined normal range, said sumincreases until it reaches the reference voltage of the first referencevoltage means whereupon said first reference voltage means clamps thevoltage across said first resistance means and thereby limits thecurrent therethrough,and upon said voltage dependent second resistancemeans detecting its threshold voltage in response to said transientabnormal high voltage surge across said terminal means, said secondresistance means permits said current to further increase through saidfirst resistance means until the voltage drop across said firstresistance means equals the sum of the reference voltages of the firstand second reference voltage means whereupon the voltage across saidfirst resistance means is clamped and the semiconductor switch means isrendered nonconducting by the second voltage reference means.
 2. Aprotection circuit as claimed in claim 1, wherein said first voltagereference means comprises two serially connected semiconductor diodes.3. A protection circuit as claimed in claim 2, wherein said secondvoltage reference means comprises a semiconductor diode.
 4. A protectioncircuit as claimed in claim 1, wherein said voltage dependent secondresistance means comprises a varistor.
 5. A protection circuit asclaimed in claim 1, wherein said controllable semiconductor switchcomprises at least one bipolar transistor whose collector/emitterjunction forms the said conductive path and whose base element iscoupled to control means.
 6. A protection circuit as claimed in claim 5,wherein said base element is coupled to a conductive path of a furthertransistor whose base element is coupled to said control means.
 7. Aprotection circuit as claimed in claim 5, wherein said semiconductorswitch comprises a complementary configuration of an NPN and a PNPtransistor, the collector/emitter junction of the NPN transistor formingsaid conductive path, the base/emitter junction of said PNP transistorforming said semiconductor junction.
 8. A protection circuit as claimedin claim 7, wherein said semiconductor switch is a high voltage bipolardevice.
 9. A protection circuit as claimed in claim 1, whereinsaidprotection circuit further comprises a polarity guard device having aconstant polarity output and an AC input, and said line terminal meansis operatively connected across said constant polarity DC output meansand said AC input is connected to said line.
 10. A protection circuit asclaimed in claim 1, wherein said controllable semiconductor switch is atelephone subset line switch whose control element is coupled to controlmeans associated with a telephone subset circuit for controllablyswitching a loop current circuit of said subset circuit.
 11. An AC powerswitch arrangement for connecting an AC power line to a load, saidarrangement comprisingline terminal means further comprising first andsecond line terminals, load terminal means further comprising first andsecond line terminals, a protection circuit as claimed in claim 1 withits respective said first resistor means and the conductive path of itsrespective said semiconductor switch coupling said first line terminalto said first load terminal, and a second protection circuit as claimedin claim 1 with its respective said first resistor means and theconductive path of its respective said semiconductor switch couplingsaid second line terminal to said second load terminal.